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Rightly or wrongly, economic data released this close to a presidential election acquire outsize importance as a judgment on the performance of the incumbent. In that light, last week's battery of numbers for August was a garden-variety example of Obamanomics -- basically positive, but for this late in the recovery, surely o-bominable.

Early in the week, the Institute for Supply Management reported a further tick down in its purchasing managers' index for August, to 49.6. Since a PMI of 50 is technically "break-even" for manufacturing, 49.6 signals a slight contraction. But that still correlates with modest growth in overall GDP.

For confirmation that the economy is still growing, the August index for the services sector, also released by the Institute for Supply Management, rose to 53.7 from 52.6 -- still at the bottom half of the range of the past two years, but solidly expansionary. Manufacturing is probably being hurt by softer sales abroad, while services mainly operate domestically.

Friday -- the very morning after President Obama portrayed himself as a job-creator at the Democratic National Convention -- the Bureau of Labor Statistics reported an increase of just 96,000 nonfarm payroll jobs in August. With downward revisions to prior months, the net gain was a meager 55,000.

The unemployment rate fell to 8.1% from 8.3% -- a possible bright spot that was dismissed by analysts as bogus, since it was accompanied by a reported decline in the labor force. I'm not so sure. Follow-ups on declines in the rate of joblessness accompanied by the declines in the labor force generally show that the labor force has a tendency to bounce back, while the fall in the unemployment rate has a tendency to confirm. In any case, broader measures of the unemployment rate -- especially the broadest measure, "U-6" -- also fell in August.

But an 8.1% rate of joblessness is still abominable by any standard. And two months into the third quarter, payroll growth is averaging just 119,000. At that rate, it will take more than three years for nonfarm payroll employment to climb back to its previous peak of January 2008.

SPEAKING OF LABOR MARKETS, a recently published study from the Economic Policy Institute cites a decline since 1973 in the share of the labor force belonging to unions as a cause of rising wage inequality. But as data available from Haver Analytics show, the union share actually began its decline 20 years earlier, in 1953. If the fall in the union share from 1953 to 1973 was not accompanied by rising wage inequality, this should at least make you wonder how the EPI study could assign it this causal role post-1973.

But then again, if you torture numbers long enough, they'll confess to almost anything. With use of multivariate analysis -- in which many causal variables are introduced -- it might be possible to insist that a decline in unionization always brings wage inequality, but through the 1950s and '60s other factors were in play that offset this factor.

In the end, the question can be resolved not by manipulation of data, but through an understanding of how labor markets actually work. The EPI study documents the unsurprising fact that workers in labor unions tend to receive a higher wage than nonunionized workers. That is the first step in understanding the equally unsurprising fact that, far from diminishing wage inequality, unions tend to increase it.

As economist Morgan O. Reynolds puts it in his essay on unions in The Concise Encyclopedia of Economics, "First, monopoly unions raise wages above competitive levels. Second, nonunion wages fall because workers priced out of jobs by high union wages move into the nonunion sector and bid down wages there." Reynolds therefore concludes that "some of the gains to union members come at the expense of those who must shift to lower-paying or less desirable jobs or go unemployed."

There is no attempt in the EPI study to refute these elementary principles of supply and demand. Instead, we find the usual arguments that unions lift wages and improve working conditions in nonunion jobs by holding out the threat that nonunion industries will unionize, and by setting a general example for wages and working conditions. At bottom such arguments assume there is no such thing as a competitive wage—that in the absence of unions and government intervention, greedy capitalists would pay workers subsistence wages, which the workers would gratefully accept.

For an understanding of how greed is beside the point, and how unfettered markets pay workers according to rising productivity, try an unusually rich and imaginative discussion in economist George Reisman's great book, Capitalism, recently made available at a bargain price on e-book readers. On other key economic principles as well, this is an indispensable work.